This invention relates to methods and apparatuses for erasing information from a magnetic recording medium and to improved electromagnets for that purpose.
In one class of methods and apparatuses for erasing information from magnetic recording media, the recording medium, which may be a magnetic tape wound about a reel, is subjected to a varying or alternating electromagnetic field to randomize the magnetic particles on the magnetic material.
In one prior art method and apparatus in this class, the magnetic tape is moved into an electromagnetic field that is applied in each of a plurality of different directions, one direction at a time, such as by first applying a vertically oriented field followed by a longitudinally oriented field. Techniques of this type are described in U.S. Pat. No. 4,730,230 and U.S. Pat. No. 4,751,608.
In another prior art technique, the magnetic tape is carried by a conveyor over a rotating electromagnet that has pole faces parallel to each other in the same plane underneath the conveyor belt. Thus, the electromagnet rotates a time varying electromagnetic field with it to cause the time varying electromagnetic field to pass through the magnetic material in the tape at a plurality of different angles. This type of prior art device is disclosed in U.S. Pat. No. 4,639,821.
Still another prior art apparatus and technique of this class includes a conveyor that carries a magnetic tape into a rotating magnetic field. The rotating magnetic field is substantially parallel with the conveyor and is in the plane of the magnetic tape. It is created by electromagnetic poles on both sides of the conveyor belt, energized in such a way that similar polarities oppose each other on opposite sides of the tape with the phases of the poles on each side of the conveyor changing in synchronism to cause the field to rotate. Thus, north electromagnetic poles face each other on opposite sides of the tape and south magnetic poles face each other on opposite sides of the tape and the north and south poles alternate with each other in the same plane on the same sides of the tape. The poles rotate in synchronism.
The prior art degaussing techniques provide erasure that is satisfactory for some purposes but do not erase to the extent desired for other applications. In general, the systems which apply vertical and longitudinal fields separately have the disadvantage of moving the energy back and forth between even and odd harmonics of the recorded signal. This reduces the effectiveness of the erasure. Rotational fields by themselves do not improve the depth of erasure to the extent needed for some applications when practiced as described in the aforementioned prior art references.